fix: Don't transpile ES2018 symbol properties

[taion]

Q	A
Fixed Issues?	#4783 (again)
Patch: Bug Fix?	👍
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	No, per #5195
Documentation PR Link
Any Dependency Changes?
License	MIT

This is a repeat of #5195 to work around the same upstream issue.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/10438/

[danez]

Seems once we update to v3 this can be undone.

[zloirock]

It could be a breaking change. If someone uses old transform and new runtime, entry point @babel/runtime/core-js/symbol/async-iterator will be missed.

[taion]

@zloirock It won't be – the runtime transform will see that Symbol gets pulled in, and it will import the full symbol polyfill, which will work as before. This was the behavior in Babel v6 with #5195.

[taion]

And, yeah, ideally we'll kill this hack for good once we upgrade to core-js v3.

[nicolo-ribaudo]

We should hardcode it in build-dist, so that it isn't removed from the package.

[taion]

@nicolo-ribaudo Hardcode what?

[taion]

The reason this isn't a breaking change is because we get the "full" Symbol polyfill rather than just the one for Symbol.asyncIterator.

In the below, note that it's now _Symbol.asyncIterator instead of _Symbol$asyncIterator. But this is fine – that's properly defined and present.

Before

var _Symbol$iterator = require("../core-js/symbol/iterator");

var _Symbol$asyncIterator = require("../core-js/symbol/async-iterator");

var _Symbol = require("../core-js/symbol");

function _asyncIterator(iterable) {
  var method;

  if (typeof _Symbol === "function") {
    if (_Symbol$asyncIterator) {
      method = iterable[_Symbol$asyncIterator];
      if (method != null) return method.call(iterable);
    }

    if (_Symbol$iterator) {
      method = iterable[_Symbol$iterator];
      if (method != null) return method.call(iterable);
    }
  }

  throw new TypeError("Object is not async iterable");
}

module.exports = _asyncIterator;

After

var _Symbol$iterator = require("../core-js/symbol/iterator");

var _Symbol = require("../core-js/symbol");

function _asyncIterator(iterable) {
  var method;

  if (typeof _Symbol === "function") {
    if (_Symbol.asyncIterator) {
      method = iterable[_Symbol.asyncIterator];
      if (method != null) return method.call(iterable);
    }

    if (_Symbol$iterator) {
      method = iterable[_Symbol$iterator];
      if (method != null) return method.call(iterable);
    }
  }

  throw new TypeError("Object is not async iterable");
}

module.exports = _asyncIterator;

[zloirock]

I mean that someone could have @babel/transform-runtime pinned to, for example, 7.3 (code sample Before) and @babel/runtime@7.4 without @babel/runtime/core-js/symbol/async-iterator entry point. And use Symbol.asyncIterator directly, not just in @babel/runtime helpers.

[nicolo-ribaudo]

We should force it to be included like we do for is-iterator:

babel/packages/babel-plugin-transform-runtime/scripts/build-dist.js

Line 22 in b2b96e0

const paths = ["is-iterable", "get-iterator"];

[taion]

Makes sense. Done.